1. Field of the Invention
The present invention relates to a variable gain type amplifier.
2. Description of the Related Art
An amplifier having a large amplification gain or amplification quantity is composed of a plurality of amplifying circuits connected tin series. The plurality of amplifying a signal step by step. Generally, each of the plurality of amplifying circuits has a gain control function. An amplifier having a gain control function is called a variable gain type amplifier.
FIG. 1 shows a conventional variable gain type amplifier. Referring to FIG. 1, a variable gain type amplifier 1 is composed of a first amplifying circuit 11, a second amplifying circuit 12 and an attenuating circuit 13.
The first amplifying circuit 11 is connected with the output of the attenuating circuit 13. The second amplifying circuit 12 is connected with the output of the first amplifying circuit 11. An input signal S1 is supplied to the attenuating circuit 13. The second amplifying circuit 12 outputs an output signal S2. A power supply voltage VDD and a control voltage VAGC are applied to the attenuating circuit 13. The power supply voltage VDD is applied with the first amplifying circuit 11 and the second amplifying circuit 12.
The first amplifying circuit 11 is composed of a transistor circuit which amplifies the input signal. The second amplifying circuit 11 is composed of a transistor circuit which amplifies the input signal. The attenuation circuit 13 is composed of a transistor circuit which attenuates the input signal.
The amplification quantity (Gp1) of the first amplifying circuit 11 is fixed, and is set to 12 dB. The first amplifying circuit 11 has a noise figure of 2 dB (NF1). The amplification quantity (Gp2) of the second amplifying circuit 12 is fixed, and is set to 12 dB. The second amplifying circuit 12 has a noise figure of 3 dB (NF2). The attenuation quantity of the attenuating circuit 13 is changed in accordance with the value of the control voltage VAGC. The attenuation quantity of the attenuating circuit 13 is varied from a range of 40 to 1 dB when the value of the control voltage VAGC is varied from a range of 40 to 1 dB when the value of the control voltage VAGC is varied from a range of 0.5 to 2.5 V. During the operation of the variable gain type amplifier 1, the power supply voltage VDD is kept constant. In this way, the gain of the variable gain type amplifier 1 is determined based on the control voltage VAGC.
Hereinafter, the relation of a total amplification quantity (Gp-A) and the total noise figure (NF-A) in the variable gain type amplifier 1 will be described with reference to the total noise figure (NF-B) of FIG. 7. When the control voltage VAGC is 2.5 V, the attenuation quantity of the attenuating circuit 13 is set to 1 dB. In this case, the total amplification quantity (Gp-A) of the variable gain type amplifier 1 is set to 23 dB. Also, the total noise figure (NF-A) of the variable gain type amplifier 1 is set to 4.7 dB.
When the control voltage VAGC is decreased to 2.0 V, the attenuation quantity of the attenuating circuit 13 is set to 6 dB. In this case, the total amplification quantity (Gp-A) of the variable gain type amplifier 1 is set to 18 dB. Also, the total noise figure (NF-A) of the variable gain type amplifier 1 is set to 7.7 dB.
When the control voltage VAGC is decreased to 1.5 V, the attenuation quantity of the attenuating circuit 13 is set to 11 dB. In this case, the total amplification quantity (Gp-A) of the variable gain type amplifier 1 is set to 13 dB. Also, the total noise figure (NF-A) of the variable gain type amplifier 1 is set to 11.7 dB.
The technique relating to distortion control of the variable gain type amplifier is disclosed in Japanese Laid Open Patent Application (JP-A-Showa 62-226705). In this reference, an attenuating circuit is arranged between two amplifying circuits. However, in this reference, only an attenuation quantity of the attenuating circuit is controlled based on a control voltage.
When a variable gain type amplifier is provided in an amplification stage of a transmitter section of a handheld phone, the variable gain type amplifier generates reception interfering wave to other handheld phones which are in the reception state in the neighborhood. The reception band noise figure (dBm/Hz) indicates the degree of the reception interference and can be expressed by Gp+NF+IM+ thermal noise. Here, Gp is a total amplification quantity of the amplifier, NF is a total noise figure of the amplifier, IM is an intermodulation distortion of the amplifier, and the thermal noise is called kTB noise, and is calculated by use of the Boltzman""s constant, temperature and bandwidth. The thermal noise is constant to be xe2x88x92173.9 (dBm/Hz) in the temperature of 25xc2x0 C.
When a handheld phone is used, it is necessary to control the magnitude of the output signal based on its position from a base station in order to save power. Such reception band noise does not cause any problem since the other users are hardly in the neighborhood, when the position for the handheld phone to be used is far from the base station. On the other hand, when the position of the handheld phone is near the base station, the probability of reception interference is high because a lot of other users are in the neighborhood. Therefore, when the position of the handheld phone is near the base station, i.e., when the transmission output is made small, the reception band noise should be made small. For this purpose, it is necessary to decrease the reception band noise when the total amplification quantity is decreased in the variable gain type amplifier provided in the amplification stage of the transmitter section.
However, as shown in FIG. 7, even when the total amplification quantity (Gp-A) is decreased in the conventional variable gain type amplifier 1, the total noise figure (NF-B) is largely increased. Therefore, the reception band noise index (TXN-B) is almost unchanged. Therefore, when the conventional variable gain type amplifier 1 is used for the amplification stage of the transmitter section in the handheld phone, the conventional variable gain type amplifier 1 gives large reception interference to other handheld phones near the base station.
Also, in the variable gain type amplifier having the structure disclosed in Japanese Laid Open Patent Application (JP-A-Showa 62-226705), the attenuating circuit is arranged between two amplifying circuits. Therefore, when the attenuation quantity of the attenuating circuit is large in order to decrease the total amplification quantity, the intermodulation distortion (IM) increase. Also, the reception band noise can""t be small when the total amplification quantity is decreased.
Therefore, an object of the present invention is to provide a variable gain type amplifier, in which reception band noise can be decreased or not changed when an attenuation quantity of an attenuating circuit is increased.
Another object of the present invention is to provide a variable gain type amplifier, which is suitable for use for an amplification stage of a transmitter of a handheld phone.
In order to achieve an aspect of the present invention, a variable gain type amplifier includes a first amplifying circuit, an attenuating circuit connected to an output of the first amplifying circuit, and a second amplifying circuit connected to an output of the first amplifying circuit. An amplification of the first amplifying circuit is adjusted based on a control voltage, and an attenuation of the attenuating circuit is adjusted based on the control voltage.
Here, it is desirable that the attenuation of the attenuating circuit is increased based on the control voltage, when the control voltage is decreased in a predetermined voltage range. In this case, the amplification of the first amplifying circuit is not substantially changed when the control voltage is changed in a first range of the predetermined voltage range. Also, the amplification of the first amplifying circuit is decreased when the control voltage is decreased in a second range of the predetermined voltage range.
Also, it is desirable that the first amplifying circuit is of a self bias type. In this case, the first amplifying circuit may include a field effect transistor and a bias of a gate of the field effect transistor may be determined based on the control voltage. Also, the first amplifying circuit may be of a source follower type.
Also, it is desirable that the second amplifying circuit is of a self bias type using a power supply voltage. In this case, the second amplifying circuit may be of a source follower type.
Also, it is desirable that the attenuating circuit is of a xcfx80 type. In this case, the attenuating circuit may include a field effect transistor and a bias of a gate of the field effect transistor may be determined based on a power supply voltage. Instead, the attenuating circuit may include a field effect transistor and a bias of a gate of the field effect transistor may be determined based on the control voltage. Otherwise, the attenuating circuit may include two field effect transistors and a bias of a gate of each of the field effect transistors may be determined based on a power supply voltage.
Also, the attenuating circuit may include an N-channel field effect transistor and a bias of a source of the field effect transistor may be the control voltage. Instead, the attenuating circuit may include two N-channel field effect transistors and a bias of a drain of each of the field effect transistors may be determined based on the control voltage.
Also, the attenuating circuit my include an N-channel field effect transistor and a drain of the field effect transistor may be grounded via a resistor and a capacitor. Instead, the attenuating circuit may include two N-channel field effect transistors connected in series via a first capacitor and a drain of a first one of the field effect transistors may be grounded via a parallel circuit of a circuit of a first resistor and a second capacitor and a second resistor.
Also, it is desirable that the attenuating circuit includes an N-channel field effect transistor and a drain and source of the field effect transistor is connected by a resistor.
In order to achieve another aspect of the present invention, a variable gain type amplifier used for a transmitter of a handheld phone, includes a first amplifying circuit operating based on a control voltage, an attenuating circuit connected to an output of the first amplifying circuit, and operating based on a control voltage, and a second amplifying circuit connected to an output of the first amplifying circuit. In this case, amplification of the first amplifying circuit is decreased or not changed when attenuation of the attenuating circuit is increased based on the control voltage.
Also, the attenuation of the attenuating circuit is increased based on the control voltage, when the control voltage is decreased in a predetermined voltage range. In this case, the amplification of the first amplifying circuit is not substantially changed when the control voltage is changed in a first range of the predetermined voltage range. Also, the amplification of the first amplifying circuit is decreased when the control voltage is decreased in a second range of the predetermined voltage range.